1. Field of the Invention
The present invention relates to a semiconductor optical device, an optical transmitter module, an optical transceiver module, and an optical transmission equipment. In particular, the present invention relates to improvement of characteristics of a semiconductor optical device including a modulator having a buried heterostructure.
2. Description of the Related Art
In recent years, along with the widespread use of the Internet, there have been demands for higher performance in speed and capacity of semiconductor optical devices, optical transceiver modules, and optical transmission equipments, which are used in an optical communication system. For example, an electro-absorption modulator (hereinafter, referred to as EA modulator) is widely used because of its advantageous characteristics of small chirp (wavelength variation) at the time of modulation, large extinction ratio, the extinction ratio being a difference between an ON level and an OFF level of an optical signal, and wide bandwidth, and further, because the electro-absorption modulator is small in size and low in cost.
The EA modulator is a modulator for modulating light by selectively applying an electric field to an active region of the EA modulator, with the use of a quantum confinement Stark effect (hereinafter, referred to as QCSE). Here, the active region is a so-called single-quantum well (hereinafter, referred to as SQW) layer, or multiple-quantum well (hereinafter, referred to as MQW) layer. Hereinafter, in this specification, MQW is assumed to include SQW in addition to general MQW. Note that, QCSE refers to an effect in which an absorption edge of light in the MQW layer is shifted to the long-wavelength side when the electric field is applied to the MQW layer.
A semiconductor optical device including the EA modulator is, for example, a modulator-integrated semiconductor optical device including an EA modulator portion and an oscillator portion, which are monolithically-integrated on the same semiconductor substrate. Continuous light, which emits from the oscillator portion by drive current injection, is optically modulated by applying an electric signal to the EA modulator portion, thereby outputting signal light.
FIG. 5A is a top view of a modulator-integrated semiconductor optical device 101 according to a conventional technology. The modulator-integrated semiconductor optical device 101 includes a modulator portion 102 and an oscillator portion 103 with a P-type electrode 124, and a waveguide portion 104. As described above, the modulator portion 102 performs optical modulation of continuous light, which emits from the oscillator portion 103.
FIG. 5B is a cross-sectional view of the modulator portion 102 of the modulator-integrated semiconductor optical device 101 according to the conventional technology. The cross section of the modulator portion 102 illustrated in FIG. 5B corresponds to a cross section taken along the broken line VB-VB of FIG. 5A.
As main structures of a semiconductor optical device, there are a buried heterostructure (hereinafter, referred to as BH structure) and a ridge-waveguide structure. The modulator-integrated semiconductor optical device 101 has the BH structure. The BH structure refers to a structure in which both sides of a mesa-stripe structure of a multilayer structure including an active layer 111, the mesa-stripe structure being formed by removing regions on the outer sides of a waveguide region, are each buried with a semi-insulating semiconductor layer. An EA modulator-integrated semiconductor optical device, which integrates an EA modulator having a BH structure, is disclosed in JP 2004-273993A.
In the modulator portion 102 of the conventional technology, on an n-type substrate 110 formed over an n-type electrode 115, the active layer 111 including the MQW layer, a p-type clad layer 112, and a p-type contact layer 113 are laminated. Here, the active layer 111 is made of an undoped (without doping process) semiconductor (intrinsic semiconductor).
Outer sides of the waveguide region of the multilayer structure are removed down to a part of the n-type substrate 110, to thereby form a mesa-stripe structure. As described above, both sides of the mesa-stripe structure are each buried with a buried layer 116, which is made of a semi-insulating semiconductor doped with impurities such as iron (Fe). Compared with the ridge-waveguide structure, the BH structure has a stronger effect of light confinement in a lateral direction. Therefore, compared with the case of the ridge-waveguide structure, a far field pattern (FFP) of the BH structure is more circular, and hence there are advantages that a fiber coupling efficiency is high and excellent radiation performance may be achieved. Therefore, the BH structure is widely used.
A passivation film 117 in a predetermined shape is formed so as to cover the upper surface of the mesa-stripe structure and the upper surface of the buried layer 116 located on each side of the mesa-stripe structure. The passivation film 117 is not formed in a part of the upper surface of the mesa-stripe structure and in parts of regions which become inclined surfaces of the upper surface of the buried layer 116, and thus a through hole is formed.
On the upper side of the passivation film 117, a p-type electrode 114 in a predetermined shape is formed, and the p-type electrode 114 and the p-type contact layer 113, which is the uppermost layer of the mesa-stripe structure, are electrically connected to each other via the through hole.
Note that, the impurities added to the p-type clad layer 112 and the p-type contact layer 113 interdiffuse with the impurities added to the buried layer 116, and hence the impurities added to the p-type clad layer 112 and the p-type contact layer 113 are diffused into the buried layer 116.